1. Field of the Invention:
The present invention in general, to an apparatus and method for separating two immiscible liquids of different densities such as oil and water and also for removing sediment therefrom, and in particular, to an apparatus and method for separating immiscible particles and solids from a flowing fluid mixture using the buoyancy principles embodied in Stokes' law.
2. Description of the Related Art:
It is often desired to remove immiscible oil particles from runoff or drainage discharge water so as to avoid polluting streams, lakes, wells, or the like. As concern increases for the environment, the various states as well as the federal government have imposed regulations governing the quality of such effluent water. Such effluent water might be seen, for instance, as surface runoff from a parking lot during periodic rainfalls.
Prior solutions for this problem have attempted to use the principles of Stokes' law, EQU F=6.pi..eta.rv.sub.0
which expresses the drag force F exerted on a sphere, such as an oil particle, of radius r moving with a rise speed V.sub.0, through a fluid, such as water, that has a viscosity .eta.. The oil particle naturally tends to rise due to the forces of gravity because it has a lower density .rho..sub.O than that of water, .rho..sub.W, thereby causing the oil particle to have buoyant forces thereupon. However, in order for Stokes' law to strictly apply, the fluid must be quiescent and non-moving, or, at best, a "creeping flow," and the particles must be substantially rigid (non-deformable) spheres. In such a situation, there is no separation of liquid from the rear of the rising sphere as it moves, and viscous effects dominate the particle's movement. The Reynolds number R.sub.O of the oil particle, governing its rise through water, is well-known to be ##EQU1## where d is the diameter of the spherical oil particle. If the Reynolds rise number R.sub.O is greater than 0.1, the drag force F on the sphere will be underpredicted and the oil particle will rise more slowly than expected. For instance, if R.sub.O is 1.0, the drag force F will be underpredicted by a factor of ten percent (10%). See R. Byron Bird, Warren E. Stewart, and Edwin N. Lightfoot, Transport Phenomena 192-94 (1960).
The preceding discussion shall be understood to apply primarily to small oil particles only, in the range of ten to twenty microns in diameter and below, which are the hardest particles to remove. Larger diameter liquid spheres will have higher terminal velocities than that predicted by Stokes' Law due to internal liquid circulation within the spheres in a manner well-known to those skilled in the art. Liquid within larger spheres at the interface between the sphere and the surrounding fluid will tend to move along with the surrounding fluid flow and recirculate back along the axis of the sphere, thereby reducing drag forces at the boundary interface of the sphere. As the spheres become even larger, they tend to deform into an "inverted teardrop" shape, further reducing the drag forces because of the more aerodynamic shape of the deformed particle. See Robert H. Perry, Don W. Green, and James O. Maloney, Perry's Chemical Engineers' Handbook at 5-63 to 5-64 (6th ed. 1984). Small oil particles, however, are substantially rigid spheres without appreciable deformation or internal circulation, and therefore are the hardest particles to remove.
Furthermore, when the speed with which an oil-water fluid mixture flows through any oil-removal apparatus exceeds a certain critical value, which depends on properties of the fluid and the channels within the apparatus, the flow of the oil-water fluid mixture becomes turbulent, not laminar. This turbulence renders Stokes' law, which describes the drag force on a particle in a quiescent fluid, inapplicable. A measure of the turbulence within a channel of flowing fluid, such as an oil-water mixture, is given by the Reynolds number R.sub.C for the channel, defined by the well-known relationship ##EQU2## where D.sub.H is the well-known "hydraulic diameter" of the channel and v.sub.AV is the average velocity of the fluid through the channel. If R.sub.C is less than 2,000, the flow is completely laminar and non-turbulent. If R.sub.C is greater than 10,000, the flow is completely turbulent and non-laminar. As R.sub.C moves between these values, the flow goes from being completely laminar to being completely turbulent.
Previous known solutions have employed various inclined planes and baffles to separate oil particles from water, but have not addressed the inapplicability of Stokes' law to a moving flow of water. Such incomplete solutions to the oil-water separation problem yield less than optimal removal of oil from the oil-water mixture, and are known to degrade significantly in performance as oil accumulates at the upper surface within the apparatus and lowers the boundary between the removed oil and the flowing oil-water mixture therebelow. Other approaches employing various filters and the like have a known tendency to clog and become blocked with sediment. Other known approaches employ centrifugal forces, unlike the present invention, to separate oil from water.
It shall be understood that, while the present application uses oil and water as an example of two immiscible liquids of differing density, the problems faced by the present invention and its novel solution are equally applicable to other immiscible liquids of differing densities (buoyancies), in a manner that those skilled in the art will readily recognize.
It is therefore desirable to have both an apparatus and method for separating two immiscible liquids of different density, and in particular, for separating particles of one such liquid from a fluid mixture with the other, that addresses these problems of turbulence and clogging and therefore improves the removal of the particulate. The apparatus should have no moving parts requiring service, should not suffer significant performance degradations as removed liquid begins to accumulate within the apparatus, and the resulting purified liquid should exceed applicable state and federal regulations governing discharged effluent.